A physiological sensor system of this type is used for the in situ recording of physiological measurement signals during an examination of a patient using a magnetic resonance device (MR device). From the measurement signals can be obtained, for example, ECG derivatives which provide information about the heart phase during the examination and permit a synchronization of MR measurement sequences and the heart activity.
Thus, as a result of continuously sensing the position of the heart it is possible to control the image recording operation of the MR device. If the magnetic resonance images should show the heart in a particular valve position for example, then it is possible by using the ECG signals to sense exactly the moment at which the heart is in the desired valve position and to synchronize the image recording by this means, in other words to trigger the recording for example.
A physiological sensor system for recording electrical measurement signals in an environment which adversely affects the recording, in particular in a magnetic resonance device, is known from DE 100 47 365 A1. It has a plurality of measuring electrodes and also a signal amplifier unit, a power supply unit and an electronics facility for signal conversion and signal transfer to an external signal processing device and/or control device, whereby the measuring electrodes and the signal amplifier unit are arranged in a first shielded housing and the power supply unit and the electronics facility are arranged in a second shielded housing. In addition, the signal amplifier unit is or can be connected to the electronics facility and the power supply unit by way of a shielded and/or twisted cable connection.
Similar physiological sensor systems are known from U.S. Pat. No. 5,782,241 and U.S. Pat. No. 6,052,614, in which all the elements relevant to the recording and preprocessing operation of the measurement signals are arranged together in a single housing which is to be placed on the patient. However, from this configuration results the disadvantage that on account of the considerable structural size and the simultaneous integration of the measuring electrodes the sensor system needs to be positioned close to the heart. The danger thus exists that this sensor system is at least partially situated in the recording area, in other words in the area in which the magnetic resonance image is to be recorded. The latter is at least adversely affected as a result.
A facility for MR tomography is known from EP 0 173 130 A1, in which the electrodes are connected by way of a cable connection with an amplifier facility situated outside the MR device. From this amplifier facility, which is arranged together with the MR device in an HF chamber, the measurement signals are sent by way of a fiber-optic connection to a processing facility situated externally with respect to the chamber.
Further facilities for acquiring ECG signals, in particular also with regard to core-spin tomography, are known from DE 696 28 354 T2, DE 34 30 625 A1 and DE 33 27 731 A1. Cable connections between body electrodes and amplifiers which contain a safety resistor are Cable connections between body electrodes and amplifiers which contain a safety resistor are set down in the publications.
In general, the electrical and magnetic fields underlying the MR measurement sequences couple into electrical conductors. Also affected here are the cable connections with the electrodes for the measurement of the ECG derivatives for example, with the result that particularly in the case of an elevated basic magnetic field strength (for example, greater than 1 T) a reliable determination of the heart phase is adversely affected if no countermeasures are undertaken. In addition, the arrangement of measuring electrode and cable connection must be designed such that an inadmissible warming of the parts coming into contact with a patient caused by the fields to be coupled in is prevented. A further requirement relating to the physiological sensor system lies in the use of non-permeable materials since otherwise disturbance to the magnetic resonance images would occur. This causes difficulties particularly with regard to the implementation of the electrode clips. These are normally used in order to establish electrical contact with single-use adhesive electrodes by using a clamping contact. Their spring effect cannot be achieved using conventional springs made of magnetic materials.
MR measurement sequences comprise high-frequency signals, which are beamed into the imaging area of the MR device in order to generate MR response signals, and also gradient fields for location coding of the frequencies and phases of the MR response signals. Currents induced by the HF signals can flow from the measuring electrode to the patient, whereby a localized warming effect can occur as a result of the resistance between measuring electrode and skin in the order of magnitude of 10 kΩ, which it is necessary to limit. This is achieved for example by means of a resistive line in the cable connection which limits the line currents and whereby the heat loss occurring is dissipated by being distributed over the entire line length. Carbon and stainless steel lines are known as resistive, non-permeable lines. Carbon lines consist of elastomers which are mixed with fine carbon and thereby become conducting. By adjusting the carbon proportion, the desired resistance value can be set in the order of magnitude of 10 kOhm/m. Stainless steel lines consist of extremely thin stainless steel wire which is wound onto a non-conducting carrier wire. The desired resistance value is on the other hand obtained as a result of the great line length achieved. Stainless steel lines are additionally very good at blocking the coupling-in of the electrical HF fields through the inductance formed by the winding. However, this also means that an increasing number of magnetic disturbances are captured as a result of the gradient fields. By contrast, carbon lines are less sensitive to magnetic disturbances. They are however more sensitive to high-frequency electrical disturbances. Since HF disturbances can be removed very well from the low-frequency ECG useful signal by means of low-pass filters, the carbon lines have the advantage over stainless steel lines. However, in the case of material transitions from a carbon line to the amplifier electronics or to the electrode clip there is a danger of a non-linear contact which results in a partial detection of the HF disturbance. These pass unhindered through the low-pass filters as an envelope curve signal and are amplified together with the ECG signal. This effect has also been observed with stainless steel lines which are manufactured using electrode clips made from carbon duroplast. Depending on the production quality of the material transitions, the level of the disturbance coupling-in can be a multiple of the QRS amplitude in the ECG signal, with the result that it becomes necessary to single out the lines of inferior quality. In addition, this disadvantageous effect can also be intensified during the course of use, with the result that the period of usability is restricted.